Introduction

The PHP Data Objects (PDO) extension defines a lightweight, consistent interface
for accessing databases in PHP. Each database driver that
implements the PDO interface can expose database-specific
features as regular extension functions. Note that you cannot
perform any database functions using the PDO extension by
itself; you must use a database-specific
PDO driver to access a database server.

PDO provides a data-access abstraction layer, which
means that, regardless of which database you're using, you use the same
functions to issue queries and fetch data. PDO does
not provide a database
abstraction; it doesn't rewrite SQL or emulate missing features. You
should use a full-blown abstraction layer if you need that facility.

PDO ships with PHP 5.1, and is available as a PECL extension for PHP 5.0;
PDO requires the new OO features in the core of PHP 5, and so will not
run with earlier versions of PHP.